1. Field of the Invention
This invention relates to an A/D conversion apparatus that carries out delta-sigma (xcex94xcexa3) A/D conversion through oversampling, and more particularly to an AD conversion apparatus of this kind that suitably carries out the A/D conversion over a wide dynamic range through control of the input gain.
2. Prior Art
A/D converters are used for conversion of an analog signal to a digital signal, and some of them employ a so-called floating method to enhance the accuracy of A/D conversion. An A/D converter employing the floating method controls the gain of an input analog signal according to a digital signal after the A/D conversion. FIG. 1 shows a conventional A/D conversion system based on the floating method.
In the figure, reference numeral 100 designates an input gain control circuit to which an input analog signal Sin is supplied. The input gain control circuit 100 controls the input gain based on a control signal C. Reference numeral 200 designates an A/D converter connected to the input gain control circuit 100. The A/D converter 200 converts an analog signal output from the circuit 100 to a digital signal. Reference numeral 300 designates a CPU provided at the next stage of the A/D converter 200. The CPU 300 loads a control program into a main memory for working, and generates the control signal C by executing the program such that the digital signal assumes values within a predetermined range.
The A/D conversion system constructed as above operates in the following manner: When the level of the input analog signal Sin becomes higher than a predetermined value, the CPU 300 detects this and generates the control signal C such that the input gain is decreased. On the other hand, when the level of the input analog signal Sin becomes lower than the predetermined value, the CPU 300 detects this and generates the control signal C such that the input gain is increased. In this way, the input gain is adjusted such that the level of the input signal to the A/D converter 200 is maintained within a predetermined appropriate range. For instance, where the input gain is switched between four stages, it is possible to realize an A/D converter with 10 bit accuracy by using the A/D converter 200 with 8 bit accuracy.
The A/D conversion system described above generates the control signal C by executing the program by the CPU 300 to maintain the level of input signal to the A/D converter 200 within the predetermined appropriate range. This brings about the following problems:
First, it is required to provide the CPU 300 and a storage device to store the program, and this complicates the construction of the system.
Secondly, since the CPU 300 carries out an arithmetic operation, it takes time to generate the control signal C, resulting in a poor response of the system. For instance, when the input analog signal rises sharply, if the arithmetic operation is not carried out fast enough, the control signal cannot be generated in a manner fully following up a change in the input analog signal, so that the waveform of the signal can be clipped.
Thirdly, if the number of bits of the output of the A/D converter 200 is increased, load on the CPU 300 is increased due to an increase in the required arithmetic operation.
On the other hand, another type of A/D conversion apparatus, which is based on the xcex94xcexa3 method, is conventionally known. FIG. 2 shows the construction of the conventional A/D conversion apparatus based on the xcex94xcexa3 method. A xcex94xcexa3 modulator 31 for converting an input analog signal into serial-bit strings is comprised of a switched-capacitor integrator 33, a one-bit quantizer 33 formed by a clocked comparator for quantizing an output of the integrator 33, and a feedback circuit 34 for feeding back an output of the one-bit quantizer 33 to the integrator 32, by delaying the output by one sample based on a positive reference voltage VREF+ or a negative reference voltage VREFxe2x88x92 selected depending upon the polarity of the output. The serial bit strings obtained by the xcex94xcexa3 modulator 31 are delivered to a digital filter 35 where low-frequency components corresponding to the input analog signal are extracted and converted into digital data having a predetermined number of bits.
To reduce noise of an output digital signal from the A/D conversion apparatus constructed as above, there has been proposed by U.S. Pat. No. 4,851,841 a scaling method which reduces the gain of the xcex94xcexa3 modulator 31 to 1/A (=1/1.25, for example), and designs the digital filter 35 to have impulse-response coefficients which are selected to provide a gain of A (=1.25, for example).
If the analog input signal contains a DC offset component, to remove the DC offset component at last, it is required that the digital filter 35 is followed by a high-pass filter 36, for example, as shown in FIG. 3. However, when providing the high-pass filter which follows the digital filter 35 under a condition where a certain gain is applied to the digital filter by employing the aforementioned scaling system, the DC offset component can be removed, but a positive or negative part of the input analog signal (actually the digital signal) can be clipped for high-level signals. Such a clipped state may cause deformation corresponding to overflow of data.
To solve this problem, the present assignee has proposed, by U.S. Pat. No. 5,757,299, a scaling method which multiplies the output of the high-pass filter which removes the DC offset component by a factor of A to compensate for the reduction of the gain of the xcex94xcexa3 modulator to 1/A, thereby effectively reducing noise while eliminating effect due to occurrence of a clipped state in the analog input signal.
It is a first object of the invention to provide an A/D conversion apparatus which is capable of securing a wide dynamic range of A/D conversion with a simple construction through suitably switching the input gain of the input analog signal between predetermined levels.
A second object of the invention is to provide an A/D conversion apparatus which is capable of effectively reducing noise of the output digital signal while securing a wide dynamic range of A/D conversion.
To attain the first object, according to a first aspect of the invention, there is provided an A/D conversion apparatus comprising an input gain control device that controls gain of an input signal based on a control signal, a xcex94xcexa3 modulator converter that carries out oversampling of the input signal having the gain thereof controlled by the input gain control device to convert the input signal to data of one bit, a detecting device that detects a peak value of the input signal based on the data of one bit, and a gain control device that generates the control signal based on the peak value detected by the detecting device in a manner such that the input signal having the gain thereof controlled falls within a predetermined range.
To attain the first object, according to a second aspect of the invention, there is provided an A/D conversion apparatus comprising an input gain control device that controls gain of an input signal based on a control signal, a xcex94xcexa3 modulator that carries out oversampling of the input signal having the gain thereof controlled by the input gain control device to convert the input signal to data of one bit, a moving average-calculating device that calculates moving average data indicative of a moving average of the data of one bit, and a gain control device that generates the control signal based on the moving average data in a manner such that the input signal having the gain thereof controlled falls within a predetermined range.
Preferably, the moving average-calculating device calculates the moving average data based on a plurality of values of the data of one bit in a manner such that shaping noise generated by the oversampling of the input signal by the xcex94xcexa3 modulator can be sufficiently suppressed, and at the same time the moving average data has a flat frequency characteristic over a bandwidth of the input signal.
More preferably, a number of the values of the data of one bit used for calculating the moving average data is set based on at least one of a frequency of the oversampling and the bandwidth of the input signal.
Preferably, the gain control device generates the control signal with a faster response in a gain-decreasing direction and with a slower response in a gain-increasing direction.
Preferably, the gain control device generates the control signal with a faster response in a gain-decreasing direction and with a slower response in a gain-increasing direction.
More preferably, the A/D conversion apparatus includes a maximum value-detecting device that detects a maximum value of the moving average data over a predetermined time period, and the gain control device generates the control signal in the gain-decreasing direction when the maximum value becomes larger than a first predetermined value.
Further preferably, the first predetermine value is set to a value smaller than a saturation level of the input signal in dependence on a slew rate of the input signal and a response of the gain control device.
Further preferably, the gain control device generates the control signal in the gain-increasing direction when the maximum value of the moving average data continues to be smaller than a second predetermined value smaller than the first predetermine over a predetermined time period.
Alternatively, the gain control device generates the control signal in the gain-increasing direction when the maximum value of the moving average data becomes smaller than a third predetermined value smaller than the second predetermined value.
Also alternatively, the gain control device generates the control signal in the gain-increasing direction when a value of the moving average data becomes smaller than a third predetermined value smaller than the second predetermined value.
Further preferably, the control device generates the control signal in the gain-increasing direction when a value of the moving average data continues to be smaller than a second value smaller than the first predetermine value.
Preferably, the A/D conversion apparatus includes a storage device that stores a weighting coefficient for weighting the data of one bit, the weighting coefficient being generated based on the control signal.
More preferably, the weighting coefficient is set in a manner such that as the gain is increased, the weighting coefficient is decreased, and as the gain is decreased, the weighting coefficient is increased.
Further preferably, the weighting coefficient is set to a reciprocal of the gain.
To attain the first object, according to a third aspect of the invention, there is provided an A/D conversion apparatus comprising an input gain control device that controls gain of an input signal based on a control signal, a xcex94xcexa3 modulator that carries out oversampling of the input signal having the gain thereof controlled by the input gain control device to convert the input signal to data of one bit, a moving average-calculating device that calculates moving average data indicative of a moving average of the data of one bit, a detecting device that detects a peak value of the input signal based on the moving average, and a gain control device that generates the control signal based on the peak value detected by the detecting device in a manner such that the input signal having the gain thereof controlled falls within a predetermined range.
To attain the first object, according to a fourth aspect of the invention, there is provided an A/D conversion apparatus comprising a xcex94xcexa3 modulator having a gain control function that controls gain of an input signal based on a first gain factor corresponding to a control signal, and carries out oversampling of the input signal having the gain thereof controlled to convert the input signal to bit stream data, a gain control device that generates the control signal representative of a peak value of the input signal based on the bit stream data from the xcex94xcexa3 modulator, and a decimation circuit that generates a multiple-bit digital signal based upon the bit stream data from the xcex94xcexa3 modulator and the control signal from the gain control device in a manner such that a second gain factor is applied to the multiple-bit digital signal to compensate for the gain of the input signal controlled based upon the first gain factor by the xcex94xcexa3 modulator.
To attain the second object, in the A/D conversion apparatus according to the fourth aspect, the xcex94xcexa3 modulator further has a third gain factor set to a fixed value (A), the xcex94xcexa3 modulator controlling the gain of the input signal based upon the third gain factor (A) and the first gain factor, and carrying out oversampling of the input signal having the gain thereof controlled to convert the input signal to the bit stream data. The decimation circuit further has a fourth gain factor set to a fixed value (1/A), the decimation circuit compensating for the gain of the input signal based upon the fourth gain factor (1/A) and the second gain factor.
To attain the second object, according to a fifth aspect of the invention, there is provided an A/D conversion apparatus comprising a xcex94xcexa3 modulator having a gain control function that controls gain of an input signal based on a first gain factor corresponding to a control signal and a second gain factor set to a fixed value (A), and carries out oversampling of the input signal having the gain thereof controlled to convert the input signal to bit stream data, a gain control device that generates the control signal representative of a peak value of the input signal based on the bit stream data from the xcex94xcexa3 modulator, a decimation circuit that generates a multiple-bit digital signal based upon the bit stream data from the xcex94xcexa3 modulator and the control signal from the gain control device in such a manner that a third gain factor is applied to the multiple-bit digital signal to compensate for the gain of the input signal controlled based upon the first gain factor by the xcex94xcexa3 modulator, a high-pass filter that removes a direct current component in the multiple-bit digital signal generated from the decimation circuit, and a gain applying circuit that applies a fourth gain factor set to a fixed value (1/A) to the multiple-bit digital signal having the direct current component removed from high-pass filter, to compensate for the gain of the input signal controlled based upon the second gain factor (A) by the xcex94xcexa3 modulator.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction of the accompanying drawings.